The mapReduce command, group command, and
the $where operator expressions cannot access certain
global functions or properties, such as db, that are available
in the mongo shell. See the individual command or
operator for details.

Change default JavaScript engine from SpiderMonkey to V8. The change
provides improved concurrency for JavaScript operations, modernized
JavaScript implementation, and the removal of non-standard
SpiderMonkey features, and affects all JavaScript behavior including
the commands mapReduce, group, and
eval and the query operator $where.

Increase performance for moving multiple chunks off an overloaded
shard. The balancer no longer waits for the current migration’s
delete phase to complete before starting the next chunk migration.
See Asynchronous Chunk Migration Cleanup for details.

In 2.4.4, MongoDB Enterprise uses Cyrus SASL. Earlier 2.4 Enterprise
versions use GNU SASL (libgsasl). To upgrade to 2.4.4 MongoDB
Enterprise or greater, you must install all package dependencies
related to this change, including the appropriate Cyrus SASL GSSAPI
library. See Install MongoDB Enterprise for details of
the dependencies.

For OS X, MongoDB 2.4 only supports OS X versions 10.6 (Snow Leopard)
and later. There are no other platform support changes in MongoDB
2.4. See the downloads page for
more information on platform support.